The biggest question in science fiction and fantasy is, "What if?" And sometimes we can't help applying this to the history of the genre itself. What if some of SF and fantasy's biggest mistakes hadn't happened?

Here are 10 of the all-time great missed opportunities in the history of science fiction and fantasy.

We had our own list of missed opportunities in the genre, and then we decided to see what you guys thought were the biggest examples. So we asked on io9's Facebook page — thanks to everyone who shared their ideas. (It's worth clicking over there to read the whole list.) Here are our, and your, choices for the 10 most regrettable missed opportunities:

10) William Gibson's Alien 3
One of the versions of the third Alien movie that was rejected in favor of the David Fincher was the screenplay by William (Neuromancer) Gibson, which you can read here. It mostly takes place on a space station orbiting Earth, and involves the aliens becoming a kind of airborne contagion that causes people to tear their own skin off and become Xenomorphs. And there's a lot of politics, involving the unscrupulous Union of Progressive Peoples (U.P.P.) trying to turn the arrival of the Sulaco to their advantage. One major plus: Ripley, Hicks and Newt appear throughout. (Thanks John, Michael and Scott!)

9) Neill Blomkamp's Halo
At least the failure of this project meant that we got the amazing District 9. But originally, Blomkamp was in line to direct a Halo movie, until Microsoft, Fox and Universal were unable to agree on some stuff. At Comic-Con promoting District 9, Peter Jackson said the failure of the film wasn't Microsoft's fault, and they were just as frustrated as he and Blomkamp were. Whoever was at fault, though, one thing's for sure — a Halo film from Blomkamp and Jackson would have been something special — just check out Blomkamp's Halo-inspired short film, at left. Too bad it'll never happen now.

8) The Star Trek/Doctor Who crossover
Okay, so this one probably was never meant to be. But when Russell T. Davies brought Doctor Who back to life in 2005, he had a lot of wild ideas — and one of them was to create a crossover somehow between Who and the then-current Star Trek show, Enterprise. Davies told the London Times, "Landing the Tardis on board the Enterprise would have been magnificent." Sadly, Enterprise was canceled before anything could materialize. Mostly I want to see Captain Jack Harkness interact with Starfleet, assuming this would have been during his tenure as a TARDIS passenger.

7) Joss Whedon's Wonder Woman
I still imagine what this could have been like — even if Whedon did dampen my enthusiasm slightly by suggesting that his Wonder Woman would have been akin to Angelina Jolie. I still can't help believing that Whedon's Wonder Woman film would have been something special, and would have proved once and for all that these legacy superheroines can be given a new lease on life. At the very least, it would have been one of the most memorable superhero films of the past decade.

6) Harlan Ellison's I, Robot
Okay, so anything would have been better than the Will Smith abomination. But Harlan Ellison's 1977 screenplay might actually have resulted in a great film. He spent a year toiling over an adaptation of Asimov's iconic short stories for Warner Bros., which owned the film rights. And his finished product, which Asimov himself approved of, took a sort of Citizen Kane approach to telling the life of roboticist Susan Calvin. Sadly, by the time Ellison was done, Warners wanted something more like Star Wars, and they demanded a lot of changes he wasn't willing to make. Ellison's screenplay finally appeared in Asimov's Magazine and in book form. (Thanks Matt!)

5) Guillermo del Toro's At the Mountains of Madness
Apart from Tom Cruise starring, everything about this film project sounded amazing. Del Toro wanted to make a hard R-rated version of H.P. Lovecraft's classic, bringing his full monster-making brilliance to bear. James Cameron was on board as a producer, and he sounded jazzed about this project in a way that he honestly never does when he talks about the Avatar sequels. Cameron described it as a "visual feast" and promised some mind-blowing effects work. (Thanks Lauren!)

4) Star Wars Episodes VII, VIII and IX.
Everybody complains about the prequels, but what about the sequels we never got? Okay, so they might have been just as bad as the prequels — but at least they would have had the advantage of us not knowing how things turn out beforehand. And if George Lucas had felt free to contradict the Expanded Universe books — and if he'd brought in some smart collaborators who were able to bring in fresh ideas — he might have been able to tell some really fascinating stories about what happens after you topple an evil Empire. Not to mention how Leia and Han tried to settle down. Too bad the window for Harrison Ford to come back as Han Solo is probably closed for good. (Thanks Nate and Kojak!)

3) Cancelling (Insert Name of Show Here).
Basically, the television networks' itchy cancellation finger is responsible for countless missed opportunities: shows that could have blossomed and grown, had they been allowed to continue. Including Firefly, Farscape, Space: Above and Beyond, Caprica, Terminator: the Sarah Connor Chronicles, a couple of Stargates, and countless others. Feel free to list all the other shows that were cancelled too soon in the comments.

2) Captain Kirk's Death
I honestly sit and think about what a wasted opportunity Star Trek: Generations represented way too often. If the film-makers wanted to launch a series of movies based on Star Trek: The Next Generation without any sort of passing-the-torch element, they should have gone ahead and done that. But once they chose to include William Shatner and a few other members of the Original Series crew, they took on a responsibility to do them justice. And they failed — not only giving James T. Kirk an embarrassing send-off, but also failing to seize the opportunity to tell an interesting story about the culture clash between the two Enterprise captains. Or what they could learn from each other. Or which one would win in a fight. Or, really, anything interesting. (Thanks, Jonathan!)

1) Not looking past Philip K. Dick for movie source material
As David puts it, a big mistake has been "ignoring all the other Golden Age SF writers to concentrate entirely on Philip K Dick for inspiration." Authors that people would like to see receive a movie treatment include Octavia Butler, Isaac Asimov, Iain M. Banks, Piers Anthony, Frederik Pohl and many others. At least, unlike most of the other items on this list, this is an oversight that could eventually be corrected.